Wailord for sure.
Joking aside, all energy moving decks are really... different. I was going to say skill intensive, but realized that would look both like tooting my own horn and wouldn't be 100% correct. They do take a lot of skill, but it's usually more skill as in having to know every single possible way you could ever win and be open to them then the usual things like knowing how to best get your deck rolling and stuff. If you've played energy moving decks, you might know what I mean. The difference between a good energy mover is seeing something really weird (like slapping a band onto your ancient trait Regirock and attacking because it hits weakness). Of course, energy moving decks aren't really that strong right now (if you're talking strictly competitive meta), so sorry if you didn't really care about what I just said.
In reality, I believe there aren't exactly, "harder," decks. All decks require skill to play. A skilled player can optimize anything, even something like a theme deck. Every deck is difficult, since you have to optimize it. Sure, maybe Night March is simpler than Golisopod on a surface level, but both take skill to play at a high level. Think of it kind of like all decks have similar skill ceilings, it's just that different decks have different skill floors. It's fairly easy to pick up a Wailord deck and have immediate success, whereas something like Archie's Blastoise you need to really think to be able to play at all. However, they both can be played at high levels.
Sorry if it sounds like I'm just repeating myself. In short, some decks may be harder to start playing, but all of them take a lot of skill to play optimally.